Transform minFraud response values to their types

Review @manygrams @disaacs 

## Background:
In BladeRunner we are using this gem. There were only couple things that was missing from the gem and this PR adds one of them. (See the other one at #1).

In Shopify we do some kind of [parsing](https://github.com/Shopify/shopify/blob/75f6d3d8c9e892f0723ca415e8208980ef18da28/lib/min_fraud.rb#L20-L59) on the response we receive from minFraud. Even though this app does a similar kind of parsing, it doesn't do the [type conversion](https://github.com/Shopify/shopify/blob/75f6d3d8c9e892f0723ca415e8208980ef18da28/lib/min_fraud.rb#L38-L54).

This PR adds the type conversion to the gem so we don't have to worry about it in BladeRunner.

Just as an aside: Is there really a queries_remaining value returned with each minfraud request? If so, we should send it to datadog so that we can track it, to avoid being surprised by running out of queries like what happened a couple of months ago.